a) Field of the Invention
The invention is directed to a process and an arrangement for determining reference positions for planar hybrid stepper motors. Hybrid stepper motors of this type are used especially in coordinate measuring devices and in machinery in which measuring tables or machine tables are to be moved in multiple coordinates.
b) Description of the Related Art
Such drive systems for tables are known from the publication "PASIM--direct drives--effective automation through parallel drive systems" by PASIM Antriebstechnik GmbH., Suhl, Germany, and enable highly accurate positioning and high displacement resolution at high speed and acceleration in the coordinates of the plane using hybrid stepper motors, known per se. Extensively wear-resistant and maintenance-free operation is achieved by using integrated air bearings. These systems are provided with a base plate serving as a stator, preferably with a soft-iron or soft-magnetic ceramic plate (DE 4300197) arranged thereon, and a rotor which is formed by a support plate having a soft-iron base body and controllable permanent electromagnets. The soft-iron plate or soft-magnetic ceramic plate is provided with a closely spaced grid of intersecting grooves on its surface. The surface of the rotor facing the stator advantageously has nozzles which are connected with an air supply unit providing for air bearing support.
In order to achieve high development of power and accuracy of movement and positioning, such hybrid stepper motors require faultless alignment of the structure of the rotor to the structure of the stator. Incorrect alignment of the structure of the rotor to the structure of the stator in planar hybrid stepper motors leads to impaired operation. For example, if angular deviations which may not exceed a certain magnitude should occur between the aforementioned structures, the stepper motor will remain capable of movement but will have low power development and poor accuracy in the movement of the rotor. But if these angular deviations exceed a certain limiting value, there will be no movement of the rotor. In hybrid stepper motors which operate by increments, the incremental step for a position is counted starting from a reference position or reference point. Because of the incremental operation of the stepper motor, it is necessary, in order to ensure perfect operation, to check the angular position of the rotor with reference to the stator and the correct reference point or reference position, in particular when starting or after problems occur. Previously, the alignment of the rotor relative to the stator and its adjustment to the reference point or reference position was effected manually or mechanically accompanied by visual inspection so that inaccuracies and, accordingly, faulty operation of the hybrid stepper motor could not be eliminated.